Auto merge of #152791 - nnethercote:rm-const-FLAGS, r=Zalathar

Remove `const FLAGS`.

*[View all comments](https://triagebot.infra.rust-lang.org/gh-comments/rust-lang/rust/pull/152791)*

The performance wins provided by these types are meagre, and I don't think they justify the code complexity they introduce.

r? @Zalathar

Author: bors